1. Field of the Invention
The invention relates to a device for filtering a fluid, particularly a liquefied plastic. More particularly, the invention relates to a filter device having a filter slide that contains at least two filter chambers, each chamber containing a filter.
2. Description of the Prior Art
EP 0 554 237 A discloses a filter device for contaminated fluids and a process for its operation. With this known device, at least one supply space for receiving an amount of fluid required during the backwashing process is connected to at least one downstream channel. A piston for displacing fluid out of the supply space is guided in the supply space, whereby, during the backwashing process, this supply space provides a free flowpath connection for the cleaned fluid with both the downstream side of the filter nest that is to be backwashed as well as with the downstream channel. The supply or storage space is filled with plastic material during normal production and, in the case of backwashing, is not only connected to the filter nest that is to be backwashed, but the displacing piston guided in the storage space also acts on the normal production channel, which leads from the filter currently in operation to the equipment or mould. Here the difficulty arises that, at the opening of the storage space into the production channel, the plastic mass that is to be pressed out of the storage space must partially flow against the flow direction in the production channel. Part of the plastic material pressed out of the storage space, however, also flows in the direction of the production channel, i.e., in the direction of the mould. Consequently, problems arise here that are difficult to control in practice.
The pressure constant mentioned in this reference is not attainable, because communicating ducts or channels, i.e., pressure chambers that are connection with each other, are involved here. Furthermore, the resistance in the direction of the backwashing opening is significantly less than that in the direction of the mould, so that a significant drop in pressure must occur at the mould itself.
WO 98/47688 describes a filter device for liquefied plastic, in which a displacing piston also presses a cleaned plastic mass back through the filter, in order to perform the backwashing process. The disadvantage of this arrangement is that valve devices, which close the backflow channel during normal operation, that is, during the production phase, are required. Such valve devices are undesirable in the processing of fluid thermoplastic material, because they can lead to blockages and, naturally, must also be heated. Decomposition of the plastic material can also occur in this area, because the valve device can never be completely cleaned.
Another disadvantage of the known device is that in order to change the filters, the entire valve device must be removed, which means that the construction of the known device is relatively complicated.
The disadvantages of these devices are avoided by the device according to DE 10 2004 036 597 B3. This reference describes a relatively complicated device, which does achieve the desired goal, namely, proposing a device with which the backwashing effect can be significantly improved, but which, at the same time, avoids the loss of mass in the main flow during backwashing, and ensures that the displacing pistons arranged in the channel are always washed with material.
DE 103 01 536 A1 discloses a flow-through sensor located in the discharge channel for a corresponding filter unit. A pressure generator located in the feed channel, namely, a pump or a throttle, is driven as a function of the measured through-flow.
EP 1 208 956 A1 discloses driving an extruder as a function of a pressure that is measured after or downstream of a conventional filter changer.